Minis are made specifically for travel retail, and also for promotional/gift purposes.
This is why you will often see Mini coffrets offered in airports and dutyfree stores, and seldom ever see them in regular retail stores.
Im usually a bit cheeky, and simply ask if they have any minis to offer as a gift at the time I am making a purchase.
Last week I bought Kenzo Power, and Chloé 2008 (for my wife) and asked if they had a mini they could spare. I was delighted to receive minis of Paul Smith London, 'F' by Ferragamo and the new Lanvin Femme.
My motto: if you ask for the world, you might just get half!